Plasma-assisted chemical vapor deposition systems are used to grow thin films on silicon wafers and other substrates. In the course of growing the film on the wafer or other substrate, the film is also deposited on other locations in the reaction chamber. Periodically, these deposits must be removed to prevent them from building up to the point where particulates are generated. Particulates can, for example, reduce microelectronic device yield, since a die on which a particle has landed during processing must normally be discarded.
It is known to remove film build-up with liquid etchants, vapor etchants and plasma etchants. Of these methods, plasma etching is favored because it provides superior cleaning rates and can be performed without exposing the reaction chamber to the atmosphere. These characteristics are required for high deposition system productivity. If the reaction chamber is being used to deposit a SiO.sub.2 film, for example, NF.sub.3 can be used as a clean gas. The NF.sub.3 plasma creates fluorine radicals which react with the SiO.sub.2 film under the influence of ion bombardment to form SiF.sub.4 and other volatile compounds.
The process gas is injected into the reaction chamber through a process gas injection system, which may include a plurality of tubes. The deposition gas tubes may be directed radially inward from the periphery of the reaction chamber, although other orientations of the tubes are possible. During the deposition process an unwanted film of the deposition material forms on the inner surfaces of the deposition gas tubes. This film must be removed during cleaning.
The conventional practice is to inject the clean gas through the same tubes that were used to introduce the process gas. A problem with this technique is that it has been found to be a very inefficient way of removing the film which forms on the insides of the deposition gas tubes.
Thus there is a clear need for a way of effectively cleaning the deposition gas injection tubes of a plasma-enhanced CVD system.
Fluorine-bearing compounds are frequently used as the clean gas. Following the clean cycle, a fluorine residue remains on the walls and other surfaces of the reaction chamber. The fluorine residue must be removed for safety reasons and to insure that the film adheres to the reaction chamber surfaces in the subsequent deposition cycle. U.S. Pat. No. 5,129,958 describes the use of a reducing gas to remove the fluorine residues, but this process requires the use of elevated reaction chamber temperature, which is difficult for many systems, and therefore takes an undue amount of time.